The Valkyrie are female figures who choose those who may die in battle and those who may live. They take the slain to Odin. In some lore, they provide alcoholic beverages to the slain (so there's that).
What politics would have them choose who lived and who died in a battle. Presumably, they don't support any one given faction, for they would simply annihilate the entirety of the opposite faction. Yet, in Norse mythology, there's no meaning to their selection (that I can find).
What is the best motivation that a Valkyrie would have for killing a proportion of the battlers on both sides?
EDIT: This is for a storyline; I must justify my Valkyrie picking off warriors both brave and cowards; this-side and that-side. In an attempt to prevent being too broad, I'll specify that the best answer is a reason that best justifies and benefits the Valkyries, Odin, and the world at large.
EDIT 2: Clarification as requested; just like the many questions about shapeshifters, vampires, and minotaurs (oh my), this is a question using popular mythology. I'm talking about Norse Valkyrie on Earth, not a distant planet or universe.